From the absolute time of Galileo to the Einstein space-time
The theory of relativity formulated by Albert Einstein, first in its restricted version and then in the general version, profoundly modified the Galilean theory of relativity and changed our concept of time and space. Although surprising, Einstein's forecasts have received numerous confirmations. Always to relativity we must the most famous equation of physics: E = mc2
The traveler's dilemma
Imagine that you are the passenger of a stationary train at the station and observe from the window another train that is waiting to leave on the near track. Suddenly we realize that this second train is moving away. Instinctively, to be sure that it is the other train to move and not ours, we will look for a reference on whose immobility we have no doubt, for example the station platform or a sign. But if it were night and only the other train was visible and not the station? In that case it would not be easy to decide which of the two trains started: motion is not a valid concept in an absolute sense, but it is always relative to the observer who considers it.
To study a moving body we need to equip ourselves with a meter and a clock, choose a reference point and a set of space-oriented axes from the point, with respect to which to measure distances; we must finally establish an initial instant with respect to which we measure time intervals. For the passenger who wants to know if his train has left or not, the station is a good reference, but if he were to describe the movement of a ball rolling on the floor of the car on which he travels would find it much more convenient to choose a firm reference to the floor.
On the ship of Galileo
We abandon, at least momentarily, the train to board a ship. This is the "great navy" described by Galileo Galilei in the Dialogue above the two greatest systems of the world. The Galilean ship is moving at a constant speed and does not suffer jolts or sharp turns. If the passengers do not look out of the portholes they can not understand from simple experiments, like throwing a marble on the floor, if the ship is moving relative to the Earth or it is still: the marble goes in a straight line exactly as it would on the mainland, because on the ship which proceeds at a constant speed following a straight trajectory the laws of mechanics retain full validity. In particular, the first principle of dynamics, or principle of inertia, is valid, and for this reason the ship is an inertial reference system (force).
Galileo formulated the laws that bind together the coordinates of the bodies in motion in the inertial references, called Galileo's transformations: they show how the spatial coordinates of a body are transformed when one moves from an inertial reference to another, and are based on hypothesis that time is a universal greatness, that is that all clocks, once synchronized, continue to walk with the same step regardless of the reference in which they are found.
The theory of relativity formulated by Albert Einstein, first in its restricted version and then in the general version, profoundly modified the Galilean theory of relativity and changed our concept of time and space. Although surprising, Einstein's forecasts have received numerous confirmations. Always to relativity we must the most famous equation of physics: E = mc2
The traveler's dilemma
Imagine that you are the passenger of a stationary train at the station and observe from the window another train that is waiting to leave on the near track. Suddenly we realize that this second train is moving away. Instinctively, to be sure that it is the other train to move and not ours, we will look for a reference on whose immobility we have no doubt, for example the station platform or a sign. But if it were night and only the other train was visible and not the station? In that case it would not be easy to decide which of the two trains started: motion is not a valid concept in an absolute sense, but it is always relative to the observer who considers it.
To study a moving body we need to equip ourselves with a meter and a clock, choose a reference point and a set of space-oriented axes from the point, with respect to which to measure distances; we must finally establish an initial instant with respect to which we measure time intervals. For the passenger who wants to know if his train has left or not, the station is a good reference, but if he were to describe the movement of a ball rolling on the floor of the car on which he travels would find it much more convenient to choose a firm reference to the floor.On the ship of Galileo
We abandon, at least momentarily, the train to board a ship. This is the "great navy" described by Galileo Galilei in the Dialogue above the two greatest systems of the world. The Galilean ship is moving at a constant speed and does not suffer jolts or sharp turns. If the passengers do not look out of the portholes they can not understand from simple experiments, like throwing a marble on the floor, if the ship is moving relative to the Earth or it is still: the marble goes in a straight line exactly as it would on the mainland, because on the ship which proceeds at a constant speed following a straight trajectory the laws of mechanics retain full validity. In particular, the first principle of dynamics, or principle of inertia, is valid, and for this reason the ship is an inertial reference system (force).
Galileo formulated the laws that bind together the coordinates of the bodies in motion in the inertial references, called Galileo's transformations: they show how the spatial coordinates of a body are transformed when one moves from an inertial reference to another, and are based on hypothesis that time is a universal greatness, that is that all clocks, once synchronized, continue to walk with the same step regardless of the reference in which they are found.
Commenti
Posta un commento